Generally, a switching-type power supply device is used as a constant voltage source for supplying a stable voltage to a load. The switching-type power supply device can also be configured as shown in FIG. 1 to allow a feedback signal to be proportional to an output current, so that it can be used as a constant current source for supplying an approximately constant current to a load.
In FIG. 1, the reference numeral 1 indicates an input terminal for receiving power from an external battery, and the reference numerals 2a and 2b indicate a pair of output terminals for stably supplying a given current to a load which is connected therebetween. A power conversion circuit 3 is connected between the input terminal 1 and one 2a of the output terminals. The power conversion circuit 3 is composed of a choke coil L1, a switching transistor Q1, a rectifier diode D1 and a smoothing capacitor C1 which are connected together to form a step-up chopper converter.
A current detection circuit 5 is connected between the other output terminal 2b and a ground as a zero potential reference point. The current detection circuit 5 is operable to detect a current IL flowing through a load 6 (hereinafter referred to as “load current”) so as to generate a feedback signal F1 proportional to the load current IL. Further, a control circuit 4 including a reference voltage source VR and an error amplifier EA1 is connected between the power conversion circuit 3 and the current detection circuit 5 to drive the power conversion circuit 3 in accordance with the signal level of the feedback signal F1 supplied from the current detect circuit 5.
On the assumption that a control IC for conventional external-commutation PWM control systems is used as the control circuit 4, the circuit illustrated in FIG. 1 is configured such that a reference voltage Vref generated by the reference voltage source VR is supplied to an inverting input terminal (−) of the error amplifier EA1, and the feedback signal F1 generated by the current detection circuit 5 is supplied to an non-inverting input terminal (+) of the error amplifier EA1.
A combination of the power conversion circuit 3, the control circuit 4 and the detection circuit 5 serves as a switching-type constant current power supply device. A capacitor CO connected between the input terminal 1 and the ground is an input-filtering capacitor.
The operation of the switching-type constant current power supply device in FIG. 1 will be briefly described. The switching transistor Q1 in the power conversion circuit 3 is turned on and off in accordance with a signal supplied from the control circuit 4 (this ON/OFF operation is performed at a frequency of about several hundred kHz). In conjunction with the ON/OFF operation of the switching transistor Q1, a current flows into the smoothing capacitor C1 through the choke coil L1 and the rectifier diode D1. Thus, the smoothing capacitor C1 is charged at a voltage greater than an input voltage supplied to the input terminal 1, and a current IL proportional to the interterminal voltage of the capacitor C1 flows through the load 6 and the current detection circuit 5. Then, a feedback signal F1 proportional to a load current IL is generated by the current detection circuit 5, and fed back to the control circuit 4.
The level of the feedback signal F1 to be provided from the current detection circuit 5 to the control circuit 4 has an magnitude proportional to an input current (=load current IL) instead of an magnitude proportional to an output voltage as in a typical switching-type power supply device. Thus, the control circuit 4 generates a pulsed signal having an ON-duty ratio proportional to the feedback signal F1 (=load current IL), according to a control logic configured by the error amplifier EA1, the reference voltage source VR and others, and supplies the pulsed signal to the switching transistor Q1. Then, the switching transistor Q1 is tuned on and off at the ON-duty ratio proportional to the load current IL. For example, if the load current IL is lower than a target stabilization value, the switching transistor Q1 is operated to increase the interterminal voltage of the smoothing capacitor C1 so as to lead the load current IL to be increased. Based on the above operation, the power supply device in FIG. 1 stabilizes the load current IL.
Recent electronic apparatuses are equipped with various sizes and types of display units and/or lighting units, and a light-emitting diode (hereinafter referred to as “LED”) is used in most of such display units and lighting units, as a light source thereof. In case where the LED is used as a light source, its characteristics, such as light intensity and brightness, can be maintained constant only if a supply current for the LED is maintained at a constant value. In order to supply a stable current to the LED, a part of recent electronic apparatus are provided with the switching-type constant current power supply device configured as shown in FIG. 1, which is associated with a display unit and/or a lighting unit thereof. Such a combination of the LED and the power supply device is disclosed in Japanese Patent Laid-Open Publication Nos. 11-068161, 2001-215913 and 2002-203988.
A part of recent display units or lighting units using the LED as its light source are designed such that the LED is repeatedly turned on and off at a speed unrecognizable to human eyes (specifically at several hundred Hz) to perform a lighting control. Such display units or lighting units inevitably have one period where a certain current flows through the LED (hereinafter referred to as “current flow period”), and another period where no current flows through the LED (hereinafter referred to as “current cut-off period”). Thus, if a power supply for supplying a current to the LED is the switching-type constant current power supply device as shown in FIG. 1, the feedback signal F1 to be supplied from the current detection circuit 5 to the control circuit 4 during the current cut-off period will have an approximately zero level.
The control circuit 4 supplied with such a feedback signal F1 acts to set a duty ratio of the ON/OFF operation of the switching transistor Q1 at a maximum value during a current cut-off period, and then set the duty ratio at a value proportional to a target current during the subsequent current flow period. In this operation, when the duty ratio is maximized during the current cut-off period, the interterminal voltage of the smoothing capacitor C1 is sharply increased up to an excessively high value. Then, during the subsequent current flow period, a load current equal to or greater than the target stabilization value flows for a relatively long time of period. That is, a phenomenon of destabilization in load current occurs in the switching-type constant current power supply device.
It is easily conceivable that such a load-current destabilization phenomenon can be prevented if the feedback signal F1 is smoothed using a capacitor with a large capacitance before it is supplied to the control circuit 4. However, in the switching-type constant current power supply device provided with a capacitor having a capacitance capable of maintaining the supply signal for the control circuit at a significant value over the entire current cut-off period, a processed signal from the control circuit 4 is meant to be an average value of the feedback signal in a relatively long time of period. Thus, if some fluctuation or change of the load current IL is caused by a periodic ON/OFF or interruption of a load (LED) or another non-interruption factor, it will not be able to quickly recover the load current IL deviated from the target stabilization value. In consequence, the load-current destabilization phenomenon will be caused by a factor other than the current cut-off period.
As above, in case where the power supply device configured as shown in FIG. 1 is used under the condition that a load is tuned on and off, a response speed of the feedback loop tracking a path consisting of the control circuit 4, the switching transistor Q1, the smoothing capacitor C1, the load 6 and the current detection circuit 5 is likely to have difficulties in following the change of the load and to result in precluding the power supply device from stabilizing the load current IL.